Improving sewage treatment efficiencies and plant capabilities has been the subject of numerous innovations in the past few decades because of burgeoning human populations and increasingly strained capabilities of existing plants. In conventional sewage treatment plants, the sewage is received and coarsely filtered through a bar screen or the like to remove relatively large objects. The sewage is then introduced into a primary clarifier, which generally comprises a tank, either circular or rectangular, in which gravity causes solid particles suspended in the liquid to settle out onto the bottom of the clarifier tank. Clarified liquid at the top of the tank flows over a weir at the tank edge into a launder, from which it is transported to further processing stages. The sludge, or settled solids at the bottom of the tank, is moved by hydraulic pressures, scraper arms, and gravity into a hopper from which the sludge is transported to further treatment stages.
The liquid wastewater effluent from the primary clarifier is then mixed with solid particles consisting primarily of micro-organisms (collectively called "activated sludge") to form an activated sludge mixed liquor. This liquor is introduced into an aeration or biological basin or equivalent structure where it is aerated with air or oxygen to encourage the growth and reproduction of the micro-organisms, which feed on and remove pollutants and organic matter from the wastewater. This biological aeration or oxidation process typically takes 2 to 24 hours.
After aeration, the liquor, now consisting primarily of water with a high admixed concentration of micro-organisms in small particulate or floc form, is introduced into a secondary clarifier.
The secondary clarifier is similar to the primary clarifier with some important differences. The solids suspended in the mixed liquor influent to the secondary clarifier (mixed liquor suspended solids, or MLSS) principally comprise micro-organisms, most of which will be collected and recycled to the aeration treatment stage. Since the solids settling to the floor of the secondary clarifier collectively are, in effect, a culture of respiring micro-organisms, they must be removed within 40 to 90 minutes of residence within the sludge blanket in order to avoid deterioration, gasification, and solids loss. At the same time, the sludge must be allowed enough residence time, at least 50 to 80 minutes, within the sludge blanket to compress and concentrate as much as possible into a thickened sludge mass to minimize the sludge volume to be recycled.
A prevalent problem in sludge removal from secondary clarifiers is that the removed sludge is unduly dilute by 1) removal before the sludge has concentrated sufficiently, and 2) "short-circuiting" of influent mixed liquor into the sludge outtake path, i.e., the influent wastewater breaks through the sludge blanket and flows directly into the hopper.
A large portion of activated sludge is recycled from the secondary clarifier to the biological basin. The more dilute this return activated sludge, the more must be recycled, creating a higher total volume of clarifier influent which reduces efficiency and strains the capacity of the treatment mechanisms. The maintenance of a high concentration of mixed liquor suspended solids in the biological basin and secondary clarifier influent while maintaining a low volume of return activated sludge depends on the production of a highly concentrated return activated sludge in the secondary clarifier.
The return activated sludge volume typically comprises on the order of 30 to 45 percent of the total clarifier influent feed volume.
Table 1 shows the effect on flux rate and efficiency of some different return activated sludge concentrations.
TABLE 1 ______________________________________ Effect of RSS on RAS/Q Ratio and Solids Flux Rate Case A B C D ______________________________________ Q (m.sup.3 /s) 0.8 0.8 0.8 0.8 MLSS (mg/L) 3,500 3,500 3,500 3,500 RSS (mg/L) 8,000 10,000 12,000 15,000 RAS (m.sup.3 /s) 0.62 0.43 0.33 0.24 Q + RAS (m.sup.3 /s) 1.42 1.23 1.13 1.04 RAS/Q Ratio 0.78 0.54 0.41 0.30 Flux Rate 429 372 341 314 (kg/m.sup.2 . hr .times. 10.sup.3) ______________________________________
Q=plant influent rate; MLSS=mixed liquor suspended solids concentration; RSS=return activated sludge concentration; RAS=return activated sludge rate; Flux Rate=activated sludge load, expressed as kg of dry solids/square meter/hour.
It is well known in the art that minimizing flux and total flow input (Q+RAS) by maximizing concentration of return activated sludge, such as in Cases C and D, increases the capacity and performance of a clarifier. Most prior art clarifier mechanisms operate in the range of Cases A and B.
Some prior art systems increase the depth of the sludge blanket to produce a more concentrated underflow of return sludge, but that tactic often results in excessive sludge inventory and subsequent loss of efficiency in removing suspended solids from the liquid effluent overflowing the clarifier.
U.S. Pat. No. 2,820,758 to Rankin (1952) describes a system of V-shaped scrapers to collect activated sludge, with uptake pipes removing the sludge collected between the scrapers. Rankin's system is effective in withdrawing a substantial portion of the sludge before deterioration, but the sludge taken through the uptake pipes has a low concentration, requiring the recycling of 75 to 125 percent of the sludge to maintain desirable mixed liquor suspended solid concentrations of 2500 to 4000 mg/L and results in a correspondingly decreasing operating efficiency. Additionally, Rankin's system is relatively inflexible in handling the changing nature of solids loading (flux) over a 24-hour period.
Another design of rapid sludge removal, described in Clarifier Design--Manual of Practice FD-8 (1985), Facilities Development of the Water Pollution Control Federation, Alexandria, Va., substitutes a rotating hollow tube collector in place of scrapers. Orifices in the tube withdraw activated sludge for return to the biological basin. This system is designed for use with relatively flat clarifier floors and possesses the same deficiencies of the Rankin system.
Most clarifiers in the prior art have used a rectangular hopper located in the floor near the center of the tank for the collection of sludge. It has been found that the sludge flow into the hopper due to the scrapers usually accounts for only 15 to 25 percent of total sludge underflow. The balance must be hydraulically driven to the hopper. Since the overlying liquid is often 0.5 to 1.5 meters above the hopper, and the concentrated sludge flow cannot accelerate sufficiently to satisfy the sludge volume being withdrawn, short-circuiting often results.
It has also been found that sludge flow is effected by the Coriolis effect of fluid flow, i.e., a gravitational effect tending to counter-clockwise spiral flow to a central discharge in the northern hemisphere, clockwise in the southern hemisphere.